Articulations of the human body are often very complex systems and no precise generic model exists to capture all the variability from one articulation to another. It is therefore necessary to use specific medical images or collection of digital patient data in order to get relevant information to develop techniques, devices and methods that will facilitate a treatment or a diagnosis. Our description focuses on the hip articulation between the acetabulum and the proximal femur although it can be easily extended to other articulations such as shoulder for example.
Structural abnormalities in the morphology of the hip can limit motion and result in repetitive impact of the proximal femoral neck against the acetabular labrum and its adjacent cartilage. Femoro Acetabular Impingement (FAI) is a pathology that can result from a decreased femoral head-neck offset (cam effect), an overgrowth of the bony acetabulum (pincer effect), excessive acetabular retroversion or excessive femoral anteversion, or a combination of these deformities. The cam impingement is generally characterized by a bone overgrowth located at the antero-superior aspect of the femur head-neck junction, which destructures the spherical shape of the femur head. The pincer impingement is generally characterized by an overcoverage located at the anterior aspect of the acetabulum rim. However, the correct and full diagnosis of this pathology is not easy to determine, especially when dealing with subtle deformities. Standard radiographic X-rays are used for the initial diagnosis and then three dimensional (3D) Computed Tomography (CT) scans or Magnetic Resonance Imaging (MRI) exams are generally performed in case of suspected FAI pathology. The processing of the 3D image remains a laborious manual task which cannot ensure accuracy and reproducibility, potentially misleading the diagnosis or the surgical indication. Moreover, even though 3D information can be extracted from such exams, the reconstructed bone volumes remain static and cannot predict with reliability the exact location of the impingement which occurs during the mobilization of the hip.
The surgical treatment of FAI aims at restoring a normal spherical shape to the femur head at the level of the bony cam lesion and restoring a normal coverage rate of the acetabular rim at the level of the pincer lesion, by removing the excess of bone. The result of this bony reshaping is the restoration of a greater range of motion of the hip, without impingement. Conventionally, the open surgical approach had initially been adopted since it provides a full exposure of the bone and direct access to the anatomy to be treated. Though, since minimally invasive procedures have grown in popularity by reducing the pain, morbidity and recovery time for patient, arthroscopic treatment of FAI has been explored in the last decade, which requires the use of an endoscopic camera and specific small instruments that can pass through various types of canulas. Advantages include minimally invasive access to the hip joint, peripheral compartments, and associated soft tissues. Furthermore, arthroscopy allows for a dynamic, intra-operative assessment and correction of the offending lesions. However, due to the depth of the joint and the reduced visibility and access, theses hip arthroscopy procedures are difficult to perform and not all surgeons feel comfortable about adopting the technique. The success of such arthroscopic interventions relies on a very meticulous intra-operative evaluation and a thorough and accurate correction of impingement lesions on both the femoral and acetabular sides, which can only be accomplished after a laborious learning curve over many cases. Failure of arthroscopic procedures for FAI is most commonly associated with incomplete decompression of the bony lesions. Another negative aspect of the arthroscopic procedures for FAI is the intensive use of intra-operative fluoroscopy imaging system to augment the visual control from the endoscopic camera by X-ray images. The fluoroscopic control enables a better localization of the instruments and assessment of the current correction, at the expense of high radiation exposure for the patient and the OR personnel.
Computer assisted surgical procedures have now been used in orthopedic surgery for over twenty years, in order to help the surgeon in performing the surgery with better accuracy and reproducibility. The main principle of computer assisted surgery and surgical navigation in particular is the tracking of surgical instruments relatively to the patient anatomy to guide the surgeon in order to achieve a precise target. Generally, a surgical navigation system includes a localization device, at least one tracker and a processor. One or more emitters are embedded in either the localization device or the tracker. One or more receivers are embedded in the other of the localization device or the tracker to detect the signals emitted by the emitters. The signals are transmitted to the processor, which computes localization data to detect the relative position and orientation of the tracker and localization device. Usually, three degrees of freedom are determined for the translation component of a tracker and three degrees of freedom for the rotation component. It is known that the localization device of a surgical navigation system can use several types of signals: optical, electromagnetic, ultrasonic, or other, depending on the most appropriate technology to be compatible with the surgical environment. Most commonly, passive reflective markers constitute trackers that are observed by a pair of stereoscopic camera that constitute the localization device. In other standard systems, emitters are made of infra-red LEDs and they are observed by at least three linear CCD cameras having cylindrical lenses. It is also common to use electromagnetic technology: one or several emitter coils constitute the localization device and several miniature coils constitute the trackers that can be attached to instruments or directly to the bones; miniature coils can track the full six degrees of freedom of a solid or reduced versions can track only five degrees of freedom (position of a point and orientation of an axis). Generally in orthopedic navigated surgery, at least one tracker is rigidly attached to the patient anatomy which is undergoing the surgical procedure, for example a bone, usually with a broach or pin mechanism. And at least one tracker is attached to a surgical instrument, for which the part to be tracked is calibrated, for example the tip of a drill. The localization device and the tracker are linked to the computer processor on which software is running to record trackers positions, register patient data and compute instruments trajectories.
The patient data may be obtained from several sources, either from pre-operative data such as 3D image from computer tomography (CT) scans or magnetic resonance (MR) exams for example, or from intra-operative digitization of the anatomy such as bone surface points digitization to build a bone surface model from statistical deformation of a reference model. The software of the navigation system processes patient data and according to the specific goal will generally compute an optimized trajectory or position for a surgical instrument, a cutting jig for example. Intra-operatively, the tracked instrument needs to be localized relatively to the patient anatomy. If the patient data is directly obtained from intra-operative anatomy digitization then both the patient reference system and the instrument reference system are known in the same coordinates system via the localization device, and the instrument can be directly navigated relatively to the patient data. However, in minimally invasive (MIS) procedures such as arthroscopy, the patient anatomy access is generally reduced and cannot allow for a direct digitization of the anatomy. Usually in such cases, intra-operative images such as fluoroscopy X-rays or endoscopic images are used to obtain intra-operative data. But in cases such as hip arthroscopy, 2D image information is generally not sufficient to achieve the required accuracy in three dimensions, and a pre-operative 3D image is usually required.
If the patient data is obtained from a pre-operative acquisition, an intermediate process needs to be performed before the navigation of the instrument relatively to the patient data. It is called registration, in order to match the pre-operative data of the patient to the reference system of the actual patient installed for surgery. This known procedure can be performed with a variety of different methods. It requires the acquisition of intra-operative patient data to be matched with the pre-operative patient data. The registration process can be based on specific paired points which are anatomical or fiducial points identified in the pre-operative data and matched with the same points digitized intra-operatively. The registration can also be based on image similarity measures between pre-operative image volume and intra-operative fluoroscopic image, using maximization of entropy, mutual information, or correlation coefficients for example. In the case of intra-operative image acquisition, the imaging system needs to be linked to the processor of the navigation system, and tracked by the localization device.
In the case of pre-operative acquisition of patient data such as CT or MR 3D image, a process is generally applied to the data to identify targets that can be anatomical structures, determine instruments trajectories or bony cutting planes or axes for example. The aim of the registration process is to be able to track the surgical instrument in the actual surgical site in accordance to a pre-operatively defined target. In all cases mentioned above, the surgical instrument can be also attached to the extremity of a robot or a haptic device that constrains the motions of the surgeon tool.
To use and apply these computer assisted surgery or navigation concepts to hip arthroscopy surgical procedures would provide a powerful solution to the problems stated earlier. In order to provide the arthroscopy surgeon with the most appropriate tool to help him/her achieving the optimal surgical result, specific computer assisted surgical devices and techniques have to be created to adapt to the specificity of hip arthroscopy environment and constraints.
From the issues described above, it can be easily understood that new specific devices and methods are needed to answer the problems and needs of hip arthroscopy surgeons, from the diagnosis and pre-operative planning to the actual surgical action.